For example, there are radiotherapy apparatuses that perform therapy by irradiating an affected part such as a tumor with a radiation ray, such as a proton ray, a heavy particle ray, or an X-ray.
When a radiotherapy apparatus irradiates an affected part with a radiation ray, the irradiation of the radiation ray is performed in consideration of a variation in the position or shape of the affected part due to the fact that the position or shape of the affected part is varied due to respiration or the like of a patient.
As an example of the radiation therapy, for example, methods (called pursuit irradiation) of irradiating an affected tissue with a radiation ray only when the affected tissue is moved to a predetermined irradiation position of the radiation ray have been put to practical use in order to irradiate the affected tissue with the radiation ray while suppressing the influence on a normal tissue of a patient. That is, in the pursuit irradiation, an affected part is intermittently irradiated with the radiation ray.
Further, methods (called tracking irradiation) of irradiating the irradiation position of a radiation ray while tracking the position of a moving affected part have been suggested. Furthermore, methods of irradiating a predicted position with a radiation ray by predicting a temporal variation (for example, a variation in the position of an affected part due to respiration of a patient) in the position of an affected part have been suggested.
For example, Patent Literature 1 discloses, for example, a method of acquiring transparent images for which subtle density differences are ensured by controlling irradiation timings for acquisition of the transparent images and activation timings for therapeutic radiation rays, a method of irradiating an affected part with the radiation ray with high accuracy and with reliability by predicting the most probable position of the affected part at the irradiation time of the therapeutic radiation ray based on time-series data, and a method of performing highly reliable irradiation control on an irradiation target by comparing the acquired transparent images of the irradiation target with a reference image for a specific evaluation factor.
Patent Literature 1: JP 2006-51199 A